JPH0454382B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION A. Industrial Field of Application The present invention relates to a circuit board in which heating elements are mounted as a planar array and are cooled by the evaporation of a film of dielectric cooling liquid on the surface of the elements. This invention relates to a circuit package of the type described above.

B. Prior Art Although circuit packages suitable for use with the present invention are generally known, the technical terminology will be explained. In some circuit packages, a thin flat support structure called a substrate or circuit card or circuit board (referred to herein as a card)
A semiconductor chip or other heating element is mounted on top. The semiconductor chips are mechanically and electrically connected to dielectric areas called solder pads on one side of the card, called the front side. The other major surface of the card is called the back (some cards also have elements attached to the back). The card is formed of a dielectric material or otherwise adapted to support electrical traces, the traces being
Typically, solder pads are connected to external contact points along the edges of the card. The card can be inserted into an electrical socket that forms part of a structure for supporting the card on a vertical surface.
Several cards are mounted on a common support structure with close front to back spacing between the cards. Typically, this common support structure includes a circuit board that carries wiring from card to card.

Semiconductor chips are thin squares or rectangles. One major surface of the chip faces the card and is referred to as the internal surface of the chip. In some circuit packages, solder balls connect the chip and solder pads, and the internal surface is only a short distance from the surface of the card. The outer major surface of the chip is exposed to the coolant, and heat from the chip module flows primarily along this surface. If 1
When one or many chips are locally surrounded, this local enclosure is typically
It is a thin square or rectangle. The heat transfer device can be attached to the external surface of the chip or can be preformed into a suitable piece. For example, it has been proposed to attach a fin structure to the back side of the chip (U.S. Patent Application No.
921902). Generally, the surface of the chip or associated structure that comes into contact with the coolant is referred to as the heat transfer surface.

The invention is particularly useful in memory chip packaging. A memory usually has several cards, each having a row and column arrangement of identical chips. Memory chips usually generate heat in small increments, and they are typically air cooled.
The present invention provides additional cooling for memory chips with a simple structure that avoids the complexities typically found in high power logic circuits. A typical circuit package for logic circuits is disclosed in US Pat. No. 3,993,123.

One circuit package of the general type mentioned above was introduced in 1968.
IBM Technical Disclosure Bulletin, March 2015, pp. 1557-1558, RC Chu's article "Winged Evaporative Cooling System"
Disclosure Bulletin'', March 1968, pages 1557
−1558, “Wick−Evaporative Cooling
System”, by RCChu). The enclosure holds within it a number of circuit boards arranged in a vertical plane with close spacing. The heating elements are mounted on the boards. The external surface of the element is covered by a wick material (of the lamp).
A dielectric liquid coolant flows at the bottom of the enclosure and covers the lowermost element and associated portions of the core material. The coolant rises through the wick by capillary action such that all elements not immersed in the liquid are covered by a film of liquid. This film evaporates from the heat transfer surface, thereby cooling the device. At the top of the enclosure is a water-cooled condenser from which vapor condenses and liquid coolant falls from the condenser. A gutter is present along the top edge of each board to capture liquid and distribute it to the core material. Therefore, the coolant moves while repeatedly evaporating and condensing. Note that operation is initiated by immersing the wick in liquid when the element is cold and free of evaporation. This action replenishes any liquid dripping from the condenser that is not captured by the gutter.

Other circuit packages of this general type are described in RC Chu, December 1985, IBM Technical Disclosure Bulletin, pp. 3205-3206, ``Vertically Extending Grooves Providing Capillary Action to Distribute Evaporating Dielectric Liquids''. "Circuit module with" (IBM
Technical Disclosure Bulletin December
1985, pages 3205−3206, “Circuit Modules
with Vertically Extending Grooves Providing
Capillary Action for Distribution an
Evaporating Dielectric Liquid”also by RC
Chu). The heating element has a small flat pan along its lower edge that receives liquid from the condenser. Capillary action causes the liquid to rise over the heat-conducting surface and form a film that evaporates.

C. Problems to be Solved by the Invention The purpose of the invention is to overcome the limitations of hand tube pumping and improve the cooling capacity for heating elements by minimizing the effect of air bubbles in the coolant flowing vertically over the chip. The goal is to provide the means to do so.

D. Means for Solving the Problems The circuit package of the present invention uses the features of the aforementioned Chu article published in 1985. That is, each element receives coolant directly from the condenser using a catch pot, which is an improvement on the catch pot of the above-mentioned article. However, there is a limit to the amount of liquid that can be supplied to the heating element (the limit of capillary pumping), and therefore there is a limit to the heat transfer capability of the device. To overcome this limitation, the present invention places a catch pan above the chips. The liquid flows by gravity through the apertures in the catch pan and then across the surface of the chip below.
The apertures in the capture pan are configured to provide high flow rates and improve heat transfer. First, an auxiliary heater or a small pump can be used to apply liquid to the area to be heated.

The problem is that the liquid film tends to separate from the back side of the chip. When boiling occurs,
Air bubbles form, rise through the membrane, and escape from the surface of the membrane. When the membrane flows downwardly across the surface of the chip at a rate high enough to provide good heat transfer, these bubbles are also swept downstream with the liquid and tend to accumulate near the surface of the chip. If enough bubbles accumulate, they force the membrane to separate from the surface of the chip. Since gas bubbles have much poorer heat transfer properties than liquids, the chip overheats when the membranes separate.

In the circuit package of the present invention, the heat transfer surface is slightly inclined with respect to the vertical. The component of gravity normal to the surface delays the onset of separation. It is theorized that the component of gravity normal to the surface of the chip is sufficient to offset the bubble force. The bevel is preferably achieved by a technique that allows the chip to be mounted on the card in a conventional manner such that the external surface of the chip is parallel to the front surface of the card.
In one embodiment, the sloped surface is formed as the bottom of a groove in the surface of the chip. The groove is deep at the top of the chip and shallow at the bottom of the chip. The lands between the grooves have the flat surface of the original chip. The grooves have the added benefit of increasing the heat transfer surface of the chip. The grooves can be formed in the surface of the semiconductor substrate of the chip, or a metal cap having these grooves can be attached to the chip or formed at a suitable location on the chip.

In a second embodiment, the heat transfer surface has parallelogram-shaped surface protrusions to give the heat transfer surface the appearance of tire grooves, thereby increasing the area of the heat transfer surface.

Furthermore, this particular surface is used to control the distribution of the liquid film on the heated surface. In the case of non-uniform heat flux surfaces (chips with different powers), more liquid is often guided to areas of higher power or higher heat flux.

The preferred angle is about 10 degrees. Calculations have shown that angles within this range have little effect on the heat transfer coefficient.

E. EXAMPLE FIG. 1 shows an enclosure 12 having conventional means for supporting typical circuit cards 13, 14 in a closely spaced vertical plane. Each card has elements including typical heating elements 15, 16 such as semiconductor chips attached to the card by pins 17 (the rightmost card 14 is given only to show the spacing between the cards). (its components are not shown). The card is circuit board 1
8 mechanically and electrically connected. Element 1
The schematic diagram for 5, 16 represents the chip or a local enclosure or package for the chip. The elements are typically rectangular when viewed from the front of the card and are arranged on the card in an approximately array of rows and columns. Main external surface 1 of the element
9 is called the heat transfer surface.

The bottom surface of the enclosure 12 forms a reservoir for the dielectric liquid. A heat exchanger 22 is present above the card; the heat exchanger preferably carries chilled water, although other suitable coolants can be used. The dielectric vapor condenses on the heat exchanger and falls from the heat exchanger as indicated by droplet 24 in the drawing. The heat exchanger is a horizontally flat structure that covers the entire card. The shape of the heat exchanger may be independent of the area occupied by the cards, and the system of baffles may distribute the condensate over all of the cards.

There are catch pans 26, 27 along the top edge of the heating element. The trapping pans 26 and 27 are installed in a flat roof (terrace shape) as disclosed in the paper by Chiu published in 1985, and the lower trapping pan 27 holds approximately the same amount of condensate as the upper trapping pan 26. I am starting to receive it. These have apertures or slits 30 along the bottom edge so that liquid falls from the catch pan onto the top edge onto the heat transfer surface 19. The liquid then flows by gravity across the heat transfer surface. The number 31 represents the above formed in the film of liquid. As explained below, the flow is made into a film thin enough to promote boiling within a narrow temperature range. The end of the catch pot is closed (not shown in the drawing) to maintain the liquid at the appropriate level 32.

Dashed line 33 indicates the outer surface of the dielectric film. The apertures 30 in the capture pan are sized to provide a membrane of appropriate thickness. Considering only the vertical length of the device, the initial film thickness (at the top of the device) must be thicker for longer devices. This is because the liquid evaporates as it flows down the heat transfer surface. Although tests done on elements of a given length have shown a complex relationship between element power and required film thickness, the initial film thickness for most heating elements is The diameter is thought to be in the range of 0.25 mm to 3.0 mm.

Liquid that does not fall toward the upper capture pot 26 falls toward the lower capture pot 27 or pool.
At the bottom end of the tip there is a drip rail 35 that prevents liquid from flowing upwardly over the internal surface of the tip by capillary action (the effect of capillary action is that it increases replenishment). harmless).

As disclosed in the 1985 Chiu article cited above, suitable means are provided for initiating the cycle when the device is cooled and for replenishing the liquid coolant that has fallen into the sump. A small pump (not shown) circulates the liquid from the reservoir to the top of the enclosure, from where it is distributed to the cards in the same pattern provided by the heat exchanger or buffling device.

The inclined chip surface 19 will now be described with reference to FIGS. 2 and 3. Heat transfer surface 19 has an angle of inclination of approximately 10 degrees. In the structure shown in FIGS. 2 and 3, an inclined groove 4 is formed on the back surface of the chip 15.
0 is cut in. Dashed line 41 in FIG. 2 represents the bottom of the groove and dashed line 42 represents the outer surface of the film of liquid. Techniques for forming grooves are generally known and include diamond sawing. The grooves have the added benefit of increasing the surface area of the heat transfer surface.

Alternatively, the structure of FIG. 2 can be formed as a separate structure attached to the chip, or can be preformed in place on the surface of the chip.

The width and depth of the groove are approximately 25.4×10 ^-3 to 127×, respectively.
10 ^-3 cm. The width between the fins or lands 44 between the grooves is also about 25.4 x 10 ^-3 to 127 x 10 ^-3 cm. These dimensions do not necessarily have to be the same;
In the figure, the groove 40 is wider than the land 44.

FIGS. 4 and 5 show an angled surface 46 that is either formed separately and attached to the chip, or preformed at a suitable location on the chip. The surface 46 is in the form of a parallelogram and has raised areas 47 that are combined in sloping rows oriented in the grooves of the tire. The height of the raised region 47 can decrease from the top to the bottom so that the outermost surface is vertical.

F. EFFECTS OF THE INVENTION In accordance with the present invention, a means is provided to overcome the limitations of capillary pumping and improve the cooling capacity for heating elements with minimal effects of air bubbles in the coolant.

[Brief explanation of the drawing]

FIG. 1 is a side view of a circuit package employing the heat transfer surface of the present invention. FIG. 2 is a side view of a heating element having sloped surfaces formed by grooves in the heat transfer surface. FIG. 3 is a front view of the structure of FIG. 2. FIG. 4 is a side view of a heating element having an inclined surface where the tire groove pattern in the raised area forms an extended surface. FIG. 5 is a front view of the structure of FIG. 4. 12...Surrounding body, 13,14...Card, 1
5, 16...Heating element (chip), 17...Pin,
18... Circuit board, 19... Main external surface of heating element, 22... Heat exchanger, 24... Drops, 2
6, 27...capture pot, 30...slit in the pot,
31...Steam, 32...Level of coolant, 33...
Dielectric film surface, 35...drip rail.

Claims (1)

[Scope of Claims] 1 (a) a heating element having a substantially thin rectangular shape and having first and second opposing main surfaces; (b) a heating element that supports the heating element; (c) an apparatus for mounting the heat generating element on a circuit card having a substantially flat surface for the purpose of disposing the heating element so that the first major surface of the heating element is generally parallel to and proximate to the surface of the circuit card; The circuit card is mounted in a generally perpendicular plane, with a second surface of the heating element relative to the first major surface, and a film of liquid coolant is flowed downwardly across the second surface. a semiconductor circuit package comprising: forming means at a slight angle sufficient to prevent a film of liquid from separating from said second major surface;